A conventional mobile communication system that employs a multicarrier CDMA is explained below. Transmitting and receiving apparatuses of a mobile communication system according to a multiple access system using a multicarrier CDMA system are described in, for example, “Comparison of characteristics between the SC (Single Carrier)/DS (Direct Spread)-CDMA, MC (Multi Carrier)/DS-CDMA, and MC-CDMA systems in the down link broadband radio packet transmission, The Institute of Electronics, Information and Communication Engineers, Technical Report of IEICE RCS99-130, pp. 0.63-70, October 1999”, and “Overview of Multicarrier CDMA, IEEE Communications Magazine, pp. 126-133, December, 1997”.
FIG. 14 shows a structure of the conventional multicarrier CDMA transmitting apparatus described in the above literatures. In FIG. 14, a reference numeral 201 denotes a serial to parallel converter (S/P), 202-1, 202-2, . . . , and 202-n denote a first, a second, . . ., and an Nscg (=n)-th subcarrier group modulation processors respectively, 203-1, 203-2, . . . , and 203-n denote multiplexers respectively, 204 denotes an inverse Fourier transform calculator, 205 denotes a guard interval (GI) adder, 206 denotes a frequency converter, and 207 denotes an antenna. In the subcarrier group modulation processors 202-1, 202-2, . . . , and 202-n, reference numerals 211-1, 211-2, . . . , and 211-n denote slot generators respectively, 212-1, 212-2, . . . , and 212-n denote copying sections respectively, 213-1, 213-2, . . . , and 213-k denote information modulators respectively, and 214-1, 214-2, . . . , and 214-n denote spread spectrum sections respectively.
FIG. 15 shows a structure of the conventional multicarrier CDMA receiving apparatus described in the above literatures. In FIG. 15, a reference numeral 301 denotes an antenna, 302 denotes a frequency converter, 303 denotes a guard interval (GI) remover, 304 denotes a Fourier transform calculator, 305-1, 305-2, 305-3, . . . , and 305-m denote common pilot extractors respectively, 306 denotes a by-subcarrier channel estimator, 307 denotes a delay unit, 308-1, 308-2, 308-3, . . . , and 308-m denote fading compensating sections respectively, 309 denotes an inverse spread spectrum section, 310 denotes a parallel to serial converter (P/S), and 311 denotes a data deciding section.
The operations of the conventional multicarrier CDMA transmitting and receiving apparatuses are explained below. Data transmission and reception between a base station and a plurality of terminals is assumed.
First, the operation of the transmitting apparatus is explained. Transmission data to be transmitted to an optional terminal is input to the serial to parallel converter 201, which converts the data into parallel data of a parallel number Nscg (that is, a predetermined integer). The parallel data reach the subcarrier group modulation processors 202-1 to 202-n respectively. All of the first to the Nscg-th subcarrier group modulation processors carry out the same signal processing for each subcarrier group. Therefore, the operation of the first subcarrier group modulation processor 202-1 is explained here, and the explanation of the operation of the rest of the subcarrier group modulation processors is omitted.
Of the parallel data output from the serial to parallel converter 201, the first data series is input to the subcarrier group modulation processor 202-1. The slot generator 211-1 divides the received data series into Ndata, and adds a common pilot symbol to the header of each of the divided data, thereby to prepare a frame of one data slot or N data slots. FIG. 16 shows a frame format of a subcarrier unit. As shown in this drawing, the data slot consists of a pilot symbol portion (that is, a known series), and a data portion.
The copying section 212-1 receives the data slot of the first subcarrier group, copies the frame by a predetermined number of subcarriers Nsub (=m), and prepares the data slots of the Nsub subcarriers. FIG. 17 shows in detail a structure of, for example, the copying section 212-1. The copying section 212-1 outputs the Nsub data slots to the information modulator 213-1. Other copying sections have structure similar to that of the copying section 212-1.
FIG. 18 shows in detail a structure of, for example, the information modulator 213-1. In FIG. 18, reference numerals 221-1, 221-2, . . . , and 221-m denote QPSK modulators respectively. The information modulator 213-1 receives the Nsub data slots, and the QPSK modulators 221-1 to 221-m carry out QPSK modulation of the corresponding data slots, thereby to prepare Nsub information-modulated subcarrier signals. The information modulator 213-1 outputs the Nsub information-modulated subcarrier signals to the spread spectrum section 214-1. Other information modulators have structure similar to that of the information modulator 213-1.
FIG. 19 shows in detail a structure of, for example, the spread spectrum section 214-1. In FIG. 19, a reference numeral 222 denotes a spread spectrum code generator, and 223-1, 223-2, . . . , and 223-m denote multipliers respectively. The spread spectrum section 214-1 spreads the spectrum of the Nsub information-modulated subcarrier signals respectively by using mutually orthogonal spread spectrum codes (which are expressed as ±1) given in advance in a plurality of terminals or other transmission channels. More specifically, the spread spectrum section 214-1 multiplies the Nsub information-modulated subcarrier signals by each spread spectrum code that is output from the spread spectrum code generator 222. For the spread spectrum codes, orthogonal codes of Walsh codes are generally used. The spread spectrum section 214-1 outputs the Nsub subcarrier signals after the spread spectrum to the multiplexer 203-1. Other spread spectrum sections have structure similar to that of the spread spectrum section 214-1.
The multiplexer 203-1 receives the Nsub subcarrier signals after the spread spectrum, multiplexes these subcarrier signals (that is, transmission signals to be transmitted to the terminals), and outputs the multiplexed subcarrier signals to the inverse Fourier transform calculator 204. At this time, the inverse Fourier transform calculator 204 receives the inputs of all the Nscg×Nsub (=Nc) subcarrier signals, which includes the multiplexed subcarrier signals obtained from the multiplexers 203-2 to 203-n, in addition to the input from the multiplexer 203-1. Other multiplexers have functions similar to that of the multiplexer 203-1.
The inverse Fourier transform calculator 204 calculates inverse Fourier transform of the subcarrier signals received, and outputs the resultant inverse Fourier-transformed signals to the guard interval adder 205.
FIG. 20 explains how the guard interval adder 205 adds the guard intervals. The inverse Fourier-transformed signals output from the Fourier transform calculator 204 are continuous signals of symbols. The guard interval adder 205 copies a portion at the end of the Fourier-transformed signal of each symbol corresponding to a time τGI, and adds that portion of the signal to the header of the signals for that symbol. The guard interval adder 205 outputs the guard interval-added signals to the frequency converter 206. In general, τGI is set larger than the spread of delayed waves on transmission lines, that is, τd, shown in FIG. 21. FIG. 21 shows one example of impulse responses on frequency selective fading transmission lines. As waves are reflected, diffracted, and scattered by the surrounding buildings and topography, these waves (that is, multi-path waves) arrive in the mobile communication system after passing through a plurality of transmission lines, and these waves interfere with each other (that is, frequency selective fading).
The frequency converter 206 carries out a predetermined frequency conversion processing to the received guard interval-added signals, and outputs the frequency-converted signals to the radio communication transmission lines via the antenna 207. FIG. 22 shows modulation signals on the frequency axis when Nscg is equal to four and Nsub is equal to eight, for example.
The operations of the receiving apparatus will be explained next with reference to FIG. 15. The frequency converter 302 receives, via the antenna 301, the signals influenced by the frequency selective fading on the radio communication lines, and converts these signals into baseband signals. The frequency converter 302 outputs the baseband signals to the guard interval remover 303.
The guard interval remover 303 removes the guard intervals from the received baseband signals, and generates the continuous signals of symbols (refer to the upper portion in FIG. 20). The guard interval remover 303 outputs the signals generated to the Fourier transform calculator 304.
The Fourier transform calculator 304 calculates Fourier transform of the signals received, and generates Nscg×Nsub (=Nc) subcarrier signals. The Fourier transform calculator 304 outputs all the subcarrier signals to the delay unit 307, and also outputs the subcarrier signal of each subcarrier to a corresponding one of the common pilot extractors 305-1 to 305-m.
The common pilot extractors 305-1 to 305-m extract common pilot portions from the received subcarrier signals respectively. The by-subcarrier channel estimator 306 adds in-phase channel estimate values of adjacent three subcarriers, thereby to obtain the channel estimate value of each subcarrier after suppressing noise component. The by-subcarrier channel estimator 306 outputs the channel estimate value of each subcarrier to the fading compensating sections 308-1 to 308-m in subcarrier unit.
On the other hand, the delay unit 307 receives each Fourier-transformed subcarrier signal, and delays each signal to adjust delays due to the processing in the common pilot extractors 305-1 to 305-m and the processing in the by-subcarrier channel estimator 306. The delay unit 307 outputs the respective delayed subcarrier signals to the fading compensating sections 308-1 to 308-m.
FIG. 23 shows a structure of, for example, the fading compensating section 308-1. In FIG. 23, a reference numeral 321 denotes a multiplier, and 322 denotes a complex conjugate number calculator. The complex conjugate number calculator 322 receives the channel estimate value in subcarrier unit, and calculates a complex conjugate number of the estimate value. The multiplier 321 multiplies the received subcarrier signal by the calculated complex conjugate number, and outputs the fading-compensated subcarrier signal as the result of the multiplication. The multiplier 321 outputs the fading-compensated subcarrier signal to the inverse spread spectrum section 309. Other fading compensating sections have structure similar to that of the fading compensating section 308-1.
FIG. 24 shows a structure of the inverse spread spectrum section 309. In FIG. 24, a reference numeral 323 denotes an inverse spread spectrum code generator, 324-1, 324-2, . . . , and 324-m denote multipliers, and 325 denotes a combiner. For example, Nsub subcarrier signals corresponding to each subcarrier group shown in FIG. 22 are handled as one unit of processing, and Nsub subcarrier signals are input to each of the multipliers 324-1 to 324-m. Each of the multipliers 324-1 to 324-m multiplies the Nsub subcarrier signals by the inverse spread spectrum code (which is the same as the spread spectrum code and which can be expressed as ±1) that is output from the inverse spread spectrum code generator 323. The combiner 325 combines the received inversely-spread Nsub subcarrier signals, and generates an inverse spread spectrum signal corresponding to the subcarrier group signals as the result of the combining. The combiner 325 outputs the frequency inversely-spread signal to the parallel to serial converter 310.
The parallel to serial converter 310 carries out a parallel to serial conversion of the received frequency inversely-spread signal. Last, the data deciding section 311 decides about the data of the converted signal, and demodulates the data.
However, the above conventional mobile communication system has the following problems.
For example, according to the conventional mobile communication system, multi-path waves passing through a plurality of transmission lines arrive at a mobile station, as waves that are reflected, diffracted, and scattered by the surrounding buildings and topography. These multi-path waves interfere with each other, and the frequency selective fading, that is a random fluctuation in the amplitude and the phase of the reception wave, occurs. Particularly, when the mobile station moves at a high speed, the fluctuation due to the frequency selective fading becomes at a high speed. Therefore, there has been a problem that it is not possible to sufficiently estimate the amplitude fluctuation and the phase fluctuation due to the fading, and the quality of the reception signal and the data demodulation precision are degraded.
According to the conventional mobile communication system, the multicarrier CDMA receiving apparatus calculates the degraded reception signal quality, and the multicarrier CDMA transmitting apparatus uses the degraded reception signal quality to control the transmission power. Therefore, there has been a problem that the communication quality is also degraded.
According to the conventional mobile communication system, when the transmission signal from the base station receives the influence of the frequency selective fading on the transmission line, a plurality of delayed waves exist depending on the states of the transmission lines. Therefore, there has been a problem that it is difficult to calculate the signal power already arrived at the mobile station as a criterion of the reception signal quality. Further, when the interference occurs due to the multiple user signals, it has been difficult to estimate in high precision the reception signal quality that takes into account this user interference.
In the multimedia mobile communications, the transmitting apparatus needs to change the spread spectrum rate or multiple values of the modulation signal and adaptively change the information speed according to the handled application and the states of the transmission lines. However, according to the conventional mobile communication system, there has been a problem that it is not possible to estimate the reception signal quality in high precision because of level fluctuations such as fading and shadowing.
It is an object of the present invention to at least solve the problems in the conventional technology.